Okay, I was following this page:
to do a split domain for migration.
What I was doing (and tell me if this is wrong, and the correct way to do it if possible) was putting a .forward file in the user directories that were moving.
So we'll use the example.com domain used in the webpage example.
mail comes to email@example.com. The MX points to mail.example.com. So the mail gets delivered to user on mail.example.com. In their home directory they have a .forward that sends the mail to firstname.lastname@example.org.
zimbra.example.com bounces with a relaying denied error.
How do I get that mail to email@example.com on the zimbra.example.com server?